Polyimides bistability

The Langmuir-Blodgett films (LB films) of polyimides deposited on substrates prove to be useful for aligning FLC molecular axes parallel to the transfer direction without rubbing. High-quality alignment promoting the appearance of bistable states has been reported [136]. 

The relationship between the molecular structures of polymer materials for alignment layers and the bistability has been discussed by Myrvold [14]. Here, the bistabilities resulting from alignment layers made from polyethylene, polyfumarate esters, polyvinyl alcohol (PVA), polyacrylonitrile (PAN) and polyimide (PI) will be overviewed according to his studies. 

Only those polyimides (1) with alkyl chains having an even number of carbon atoms show good bistability. This clear odd-even effect can be explained by using a picture in which the alkyl chains exist in the most outstretched, all-trans-conformation. For polyimides with alkyl chains with an odd number of carbon atoms, the polymer chain is bent and good packing is difficult. On the other hand, an even number of carbon atoms gives extended, linear alkyl groups. It can be easily understood that molecular structures of methylene derivative are odd and... 

Electrooptic bistability of a Ferroelectric Liquid Crystal Device prepared using charge transfer complex doped polyimide orientation films. Figures 1 and 2, K. Nakaya, B.Y. Zhang, M. Yoshida, I. Isa, S. Shindoh and S. Kobayashi, Japanese Journal of Applied Physics, 28 (1), p. L116 (1989). Reproduced by permission of Institute of Pure and Applied Physics. 

Enhancement of bistability by doping polyimide alignment layers with a charge transfer complex (CTC) and an appropriate rubbing process has been reported [34]. 

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